A two-component synthetic-resin molding system normally has a pair of supplies each containing a respective synthetic-resin component capable of reacting and hardening with the other component, for example a polyol and an isocyanate for making polyurethane. Each of these supplies is provided with a respective pump such as described in my copending application Ser. No. 938,011 filed Aug. 30, 1978, whose entire disclosure is herewith expressly incorporated by reference. Each of these pumps delivers predetermined metered quantities or doses of the fluid from the respective supply to the upstream end of a conduit whose downstream end is normally connected to a mixing chamber where the two components are blended. From the mixing chamber the mixed components, which are reacting, are fed to respective molds. My copending application Ser. No. 946,602 filed Sept. 28, 1978, whose disclosure is also herewith incorporated by reference, describes further details of the mold-filling procedure.
It is essential that the proportions of the components be very exact in the mixture that is used to mold the objects, otherwise the physical properties of the molded object once cured will be altogether different from those desired. As the components are normally relatively viscous, and frequently either under high pressure or vacuum, pumping of them is an extremely difficult task, especially when compounded by the necessity of dosing them very exactly. To this end it is standard practice to provide the two supplies relatively close to each other, to mount the pumps directly on the supplies, and to drive the two pumps with the same motor. Thus assuming the strokes of the pumps, which are normally of the reciprocal type that deliver a predetermined volume with each stroke, are appropriately proportioned, one is normally ensured of obtaining the right proportions in the mixture at the downstream ends of the conduit.
It is only possible to ascertain that the mixture is, indeed, a correct one once the objects molded have cured. Thus if, for instance, the checkvalve of one of the pumps fails so that this pump delivers more or less fluid than it should with each stroke, this failure will only be noticed substantially later when a good deal of the resin has been wasted and quite a few objects have been molded. Similarly if there is some leakage of the system, only at a later time will this leakage become noticeable as a product failure.
Another disadvantage of this system is that it is necessary to mount the two supplies and their pumps very close to each other so that they can be jointly driven. Such mounting makes servicing of the equipment relatively difficult, and frequently complicates filling of the two supplies.
Another disadvantage of these systems is that the reciprocating pump normally used delivers the mixed components to the mold in pulses or spurts. During the return stroke of each of the pumps the mixture is not displaced, so that production speed is held down.